Obtaining voltage, switching and timing measurements from currently manufactured CMOS and various IC's is now a standard procedure in debug and failure analysis of these complex devices. Since the introduction of flip-chip packaging technology, where access to the internal device structures is provided through the backside of the DUT, two optical methods, both non-destructive in nature, are typically used for measuring the electrical activity in IC's.
The first is known as Picosecond Imaging Circuit Analysis (PICA). Here a NIR sensitive camera or optical detectors such as avalanche photo diode is used in conjunction with the appropriate imaging optics to capture photons emitted by a circuit element as it switches logic states. The time-dependent light emission is used to obtain time resolved images of the switching events.
The second prevalent methodology is Laser Voltage Probing (LVP). LVP appears the currently preferred method for acquiring voltage and timing data from IC's. There are several improvements to LVP method that have been made. One such scheme employs two separate laser pulses which are focused to the same spot in DUT. One called the probe beam is used when the electrical circuit is active, and the other, called the reference pulse, is applied when the circuit is off. The two beams are displaced in time with respect to each other, but use a common optical path and sample the same physical location on the DUT. This cycle is repeated and the ratio of these two measurements taken again. By averaging multiple sets of ratios, the noise is reduced as compared to fluctuations inherent in a single measurement taken alone. FIG. 1 is a schematic illustration of the principles of one embodiment this technique. A laser light source 100 generates an incident beam 101. The incident beam passes through beam steering optics 150 that include a beam splitter cube 105 and a beam scanning module 115. The beam splitter cube 105, e.g., a polarizing beam splitter, diverts a portion of the incident beam to be sensed by an incident beam detector 110. The main component of the beam passes through the beam splitter cube 105 and into the beam scanning module 115. The beam scanning module may be programmed to raster scan or vector point the beam over a region of interest 121 on the device under test DUT 120. Probe beam optics 125 typically serves to focus the incident beam on an active area of the DUT, e.g., a switching gate of an IC. The incident beam, which in this case is also the probe beam, interacts with the electrically active region of the DUT and is modulated in amplitude as the voltage across the junction changes. Upon reflection, the returning laser light which now carries information encoded by the DUT, is captured by the probe beam optics 125 and relayed back along the incoming path. Upon arrival at a signal detector 130, the optical signal is converted to an electrical output, e.g., by an appropriately chosen fast photodiode and in conjunction with analog to digital conversion electronics, the signal is further processed by an oscilloscope 135 or similar signal processing electronics displaying an averaged voltage waveform. A synchronization circuit 140 handles various timing functions between the oscilloscope, the laser source and a test pattern generator 145. Further details on this system are described in U.S. Pat. No. 5,905,577, which is incorporated herein by reference.
A further refinement of the preceding technique is shown in FIG. 2. Since the incident laser beam at the DUT not only undergoes amplitude changes but phase modulation as well, a Michelson type interferometer 200 is used to capture this additional phase information as a change in amplitude that can be measured. This scheme is sometimes referred to as Phase Interferometric Detection (PID) mode. In this mode, a portion of the incident laser beam is picked off by a beam splitter 210. This portion is referred to as the reference beam. The interferometer 200 further includes a reference arm containing a lens 220 and mirror 230. The remaining portion of the incident beam is directed to a specific area of interest on the DUT. This portion is sometimes referred to as the probe beam. On reflection from the DUT the probe beam is modulated by the response of the DUT. The light beam 250 reflected by the DUT and the light beam 260 reflected by the reference arm mirror 230 are then spatially combined into the return beam 270 that now contains interference effects. The interference effects convert relative phase differences between the reflected beam from the DUT 240 and the reference arm beam 260 into amplitude differences in the combined return beam 270 which can be detected by a photo detector. Further details of the technique illustrated in FIG. 2 may be found in U.S. Pat. No. 6,496,261, which is incorporated herein by reference.
Another variation on the LVP method is called Polarization Differential Probing (PDP). Here the incident laser beam is divided into two beams each having orthogonal polarization with respect to the other. One of the polarized beams is used as a reference, while the other is designated the probe beam. Both beams are superimposed on each other, and follow a common path to be simultaneously focused onto the same location on a DUT. As shown in FIG. 3, a linearly polarized laser beam 300 is incident upon a polarization rotator 310 that rotates the polarization of the beam through some chosen angle to enter beam dividing and recombining optics 320 to provide two orthogonally polarized, but superimposed beams 325 and 330. Both beams follow a common path through beam pointing optics 335 where they are directed to be simultaneously incident on the same spot on the DUT 340. The interaction of the DUT with the laser beams is somewhat polarization dependent, and the phase of each is modulated differently according to the DUT test signals. The reflected light which contains this modulated component then retraces its incoming path and is made to interfere where the difference in phases converted to amplitude and sensed by detectors. Two separate detectors are provided to collect the orthogonal components from the two polarized beams. The signals are then passed on to collection electronics and a signal analysis system to extract the desired data. Further details of this technique may be found in U.S. Pat. No. 7,659,981 B2, which is incorporated herein by reference.
Yet another conventional technique used for phase detection, sometimes called Spatial Differential Probing (SDP) is illustrated in FIG. 4. A laser beam 405 is split into two component beams 420, 430 having mutually orthogonal polarization, e.g., by a Wollaston prism, which is located within beam manipulation optics 410. The two beams 420 and 430 have orthogonal, linear polarization states shown by dots in beam 420 and arrows in beam 430. One beam, e.g. 420, is directed to a first region 421 of the DUT, e.g., an active device region, while the other beam, e.g., 430 can be directed to a second region 422. Upon reflection of the two beams, beam manipulation optics 410 recombines to two reflected beams and converts them to the same polarization state so that they may interfere with each other to generate amplitude modulated resultant beam 440. The beam manipulation optics 410 may also include elements to provide phase offset and/or recombination of the returning beams. The phase noise due to DUT vibrations is reduced in this scheme because both beams are modulated similarly.
In this arrangement the separation between the beams is fixed. Since the geometry of various DUTs is not standard and depends upon its internal design and by the manufacturers' choices, a practical system must offer adjustability for separation of the beams. Also since the reflectivity of the area where the beams are placed can differ, a practical system must have adjustability of power for both beams to obtain best results. Examples of such systems are described, e.g., in U.S. Pat. Nos. 5,872,360, 7,616,312 B2, and 7,659,981 which are incorporated herein by reference.
A major difficulty remains with all laser based probing systems in that the signal is weak and needs separation from residual noise. The typical modulated intensity lies in the range of 100 to 200 parts per million (˜0.01%), requiring considerable time and instrumental capacity for signal averaging. To acquire a waveform with good edge definition, in practice takes from several minutes to an hour or more depending on the DUT design. Tying up equipment for such a long time places a considerable constraint on the output capacity of a semiconductor test facility.
It is within this context that aspects of the present disclosure arise.